Once the largest and arguably most important herbivore in intertidal systems along much of the west coast of the United States, the intertidal black abalone Haliotis cracherodii, has experienced mass mortalities along the coast of California since the mid-1980s. Mortality is due to infection by a pathogen that leads to a fatal wasting disease called "withering syndrome" where the foot of the abalone shrinks until it can no longer adhere to the substratum (Figure 1). Scientists first noted massive die-offs due to withering syndrome on the Channel Islands in 1986, and by 1992 it was observed near Point Conception on the mainland. The general pattern of mortality, once die-offs start, is that within a few months to a year the population will decrease by more than 90 percent, but a few remnant individuals will remain healthy and persist. Since the early 1990s the disease has migrated sequentially northwards along the California coast; this migration poses a potential threat to healthy populations of H. Cracherodii currently residing within the Monterey Bay National Marine Sanctuary.

Researchers with the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) project at UC Santa Cruz and their collaborators * are surveying black abalone populations at several sites along the south-central coast of California. Six of the sites are within the Sanctuary and are currently the only sites in the study with healthy populations (Figure 2; click here to enlarge).

Figure 2: Map of sampling sites indicating health of stock.

There are two main objectives of the surveys that started in southern California in 1992: to determine whether withering syndrome and associated mass mortalities of black abalone are progressing northward; and to assess whether the pattern of black abalone declines relates to elevated seawater temperatures. As movement of the disease northward became more evident over time, the study expanded to include more northerly sites. The study started in 1992-93 with four sites (Government Point, Boathouse, Stairs, and Purisima Point) and added three new sites in 1996-97 (Cayucos, Piedras Blancas, and Point Sierra Nevada). In 1999 five more sites were set up inside the Sanctuary: Mill Creek, Andrew Molera, Mal Paso, Rancho Marino and Point Lobos.

Sampling occurs in the early spring and late autumn. Methods consist of thorough searching for abalone in defined areas and subsequent observation for clinical evidence of the withering disease while noting recent accumulations of abalone shells. Sea surface temperatures come from NOAA's on-line database CoastWatch.

The surveys reveal that mass mortalities of H. Chracherodii due to withering syndrome are indeed progressing northwards (Figure 3; click here to enlarge). The disease had decimated populations of black abalone at the three southern-most sites by 1995. By 1998 massive die-offs struck the Purisima Point and Cayucos Point populations. Preliminary sampling in 2001 at Rancho Marino near the Sanctuary's southern border (data not included here) shows signs of the syndrome. This pattern suggests that the disease is gradually moving northwards but the spreading rate of die-offs is variable. This variable rate of decline and an apparent lag between the incidence of withering syndrome in a local population of abalone and its subsequent mass mortality due to the disease suggest that the mass mortalities may not be simply due to the gradual northward progression of the causative agent of withering syndrome. Researchers predict that elevated sea surface temperatures due to El Niño (and other causes) may act as a trigger mechanism for mass mortalities at sites where the causative agent is already present. The field evidence generally supports this prediction, but massive dies-off have occurred during El Niño and non-El Niño years. Therefore elevated seawater temperatures are seemingly not necessary for occurrence of withering syndrome and the onset of mass mortality, but likely to promote these conditions.

The prognosis for rapid natural recovery of black abalone populations along the southern and central coasts of California is not good. Black abalone along the central and extending into the northern coast of California already show signs of withering syndrome, therefore mass mortalities throughout the Sanctuary are likely. In 1999 National Marine Fisheries Service (NMFS) listed H. cracherodii as a candidate species for protection under the Endangered Species Act. If NMFS lists black abalone as endangered in the future, a management conflict may arise between protecting the remnant abalone at affect locations and the endangered sea otter population that feeds on them.

* Much of this work started prior to the founding of PISCO. Other funding sources for this research include the Natural Sciences and Engineering Research Council of Canada, the Mineral Management Service, the UC Toxics Substances Research and Teaching Program, and the National Science Foundation.